This invention relates to a modem line connection circuit and the control circuit, and more particularly to, a modem line connection circuit that is suitable to connect with public telephone line and the control circuit.
Conventionally, modem line connection circuits use a method of using a wet transformer where direct current of public telephone line is allowed to flow directly to the transformer, and a method of using a dry transformer where direct current is prevented from flowing directly to the transformer by using a simulated inductance.
At present, the method of using a dry transformer, in which it is easy to miniaturize the mounting circuit, is used typically.
FIG. 1 shows a line connection circuit to use a dry transformer. In this line connection circuit, a connection line 31 is disposed between a connector 30 and a transformer 36. Also, a series circuit composed of a connection line 32, a line switch 33 and a capacitor 35 is disposed between the connector 30 and the transformer 36. Further, a simulated inductance circuit 80 is disposed between the connection line 31 and a connection node N3 for the line switch 33 and the capacitor 35. Capacitor 34 is located between line 31 and capacitor 35.
FIG. 2 shows the details of the simulated inductance circuit 80. Terminal B1 of a diode bridge 11 for rectifying polarity inversion current of line is connected to the connection line 31, and terminal B2 of the diode bridge 11 is connected to the connection line 32. The cathode of a Zener diode 13 is connected to terminal B3 of the diode bridge 11, and the anode of the Zener diode 13 is connected through a resistance 17 to terminal B4 of the diode bridge 11. Also, a resistance 15 is parallel connected to the Zener diode 13, and a capacitor 14 is parallel connected to the resistance 17. Further, the anode of the Zener diode 13 is connected to the base of a Darlington-connected transistor Tr1, and the collectors of Darlington-connected transistors Tr1, Tr2 are connected to the cathode of the Zener diode 13. Also, the emitter of Darlington-connected transistor Tr2 is connected through a resistance 18 to terminal B4 of the diode bridge 11.
When the modem line connection circuit thus composed is connected to public telephone line, excess current is not likely to flow through the modem line connection circuit since in the public telephone line the length of line is elongated due to the connection up to the exchange office and therefore the component of DC resistance is large.
However, in PBX digital line, the length of line is short and the component of DC resistance is therefore small because there exists only the interior wiring. Therefore, when the above modem line connection circuit is connected to such a PBX digital line erroneously, excess current is likely to flow through the modem line connection circuit. Also, the form of connector to connect with the modem device is similar each other, therefore it is difficult to find the difference. Thus, there is a problem that when the modem line connection circuit is carelessly connected not to public telephone line but PBX digital line, excess current may flow in use and the mode line connection circuit may be therefore destroyed.
Accordingly, it is an object of the invention to provide a modem line connection circuit and the control method that, even when the circuit is connected to PBX digital circuit by mistake, can be prevented from being destroyed and the user can be promoted to stop the use.
It is a further object of the invention to provide a modem line connection circuit and the control method that even when the circuit is connected to PBX digital circuit by mistake, can be prevented from being destroyed and the operation of circuit can be stopped automatically to prevent the failure of circuit surely.
According to the invention, a modem line connection circuit for connecting with public telephone line, comprises:
a simulated inductance circuit composing a dry transformer;
a current-limiting circuit which limits excess current to flow through the simulated inductance circuit; and
an indicator circuit which detects the flow of excess current in response to the operation of the current-limiting circuit and indicates the detection of excess current.
According to another aspect of the invention, a control method for a modem line connection circuit for connecting with public telephone line provided with a simulated inductance circuit composing a dry transformer, comprises the steps of:
detecting excess current to flow through a Darlington circuit composing the simulated inductance circuit and then limiting the excess current up to a predetermined current; and
indicating the flow of excess current when the excess current is detected.